Querying based on request params in Craft 3

filter.twig

{# Set up the request params and queries. #}
{% set req = craft.request %}

{% set params = ['date', 'category'] %}
{% set query = {} %}
{% set search = [] %}

{% for param in params %}
    {% set key = param | camel %}
    {% set value = req.getParam(param) %}
    {% set query = query | merge({ (key):(value) }) %}
{% endfor %}

{% if search|length %}
    {% set results = results.search(search|join(' ')) %}
{% endif %}

{# ADD QUERY FILTERS HERE (see other files) #}

<form id="filter_form" action="{{ url('blog') }}" class="grid-x grid-margin-x">
    <div class="cell medium-6 large-4 xxlarge-3">
        <div class="styled-select">
            <select name="date">
                <option value="" disabled selected>Date</option>
                <option value="">All</option>
                <option value="2018" {% if query.date and query.date == '2018' %}selected{% endif %}>2018</option>
                <option value="2017" {% if query.date and query.date == '2017' %}selected{% endif %}>2017</option>
            </select>
        </div>
    </div>

    <div class="cell medium-6 large-4 xxlarge-3">
        <div class="styled-select">
            <select name="category">
                <option value="" disabled selected>Category</option>
                <option value="">All</option>        
                {% for cat in craft.categories.group('category').all() %}
                    <option value="{{ cat.title }}" {% if query.category and query.category == cat.title %}selected{% endif %}>{{ cat.title}}</option>
                {% endfor %}
            </select>
        </div>
    </div>
</form>

filterSubmit.js

filterForm() {
  let form = document.getElementById('filter_form');
  let selects = form.querySelectorAll('select');

  selects.forEach(el => {
    el.addEventListener('change', e => {
      e.preventDefault();

      form.submit();
    });
  });
}

query.category.twig

{% if query.category %}
    {% set search = search|merge(['category:' ~ query.category]) %}
{% endif %}

query.postDate.twig

{# filter the results based on a year being passed. I.e. all blogs in 2018. #}
{% if query.date %}
    {% set results = results
        .postDate('>= ' ~ query.date)
        .postDate('< ' ~ (query.date + 1)) 
    %}
{% endif %}